Electrically operated grinding mills



J. E. WILLIAMsQN 2,952,414 ELECTRICALLY OPERATED GRINDING 4MILLS 4sheets-sheet 1 nl" wml om Sept. 13, 1960 Filed Agg. 21. 1957 n, JQQLZOQh. LESS .N

Sept. 13, 1960 1. E. WILLIAMSON ELECTRICALLY OPER'ATED GRINDING MILLS 4Sheets-Sheet 2 Filed Aug. 2l, 195'? QSQ wutwwk Smm 'IAIII i2 QKQQ EBEE,I l I I I I I l l I I lll I. l I I I I I I I. I mm. Irv Q mm |r ummm@ mlll" nmN

Sept.4 13, 1960 .1.A E. wlLLlAMscN 2,952,414

ELECTRICALLY OPERATED GRINDING vMILLS filed Aug. 21, 1951 i Y 4sheets-sheet s Sept, 13, 1960 J. E. WILLIAMSON ELECTRICALLY OFERATEDGRINDING MILLS Filed Aug. 21, 1957 4 Sheets-Sheet 4 United States "atentf' John Earlston Williamson, Petersiield, Union of South Africa,assignor to Union Corporation Limited, Johannesburg, Union of SouthAfrica Filed Aug. 21, 1951, ser. No. '679,502

Claims priority, application Union of South Africa Aug. 31, 1956 Thisinvention relates to electrically operated tumbling grinding mills, suchas for example, tube mills using a coarse component of the ore as thegrinding medium.

It is a well-known fact that maximum grinding work will be accomplishedin a tumbling grinding mill when the power drawn by the mill is at amaximum, provided that other factors remain constant. 'Ihe maintenanceof maximum power input to the mill is directly dependent on maintainingthe optimum grinding medium load in the mill and, insofar as grindingmedium wear is a continuous process, the maintenance of the optimumgrinding medium load calls for constant control of the addition ofgrinding medium.

The input power to the mill motor increases with the addition ofgrinding medium, until maximum power input is reached at the optimumgrinding medium load. After the maximum is reached, power inputdecreases with further addition of grinding medium.

Manual control of the addition of grinding medium is a diiiicultoperation for the following reasons:

(a) The best power meters are not suiiiciently sensitive to give aprecise Visual indication as to when the input power is` approaching itsmaximum value.

(b) An ambiguity arises in power reading due to the decrease of inputpower if the addition of grinding medium continues after maximum powerhas been reached.

The mill input power can also be varied by varying the rotational speedof the mill.

An object of this invention is to provide improved operational controlof tumbling grinding mills.

According to one feature of the invention there is provided a method ofeffecting operational control of a tumbling grinding mill driven by anelectric motor, wherein variations in the volume of grinding medium inthe mill cause variations in the input power to the mill motor,comprising the steps of generating a control signal which is responsiveto the variations of the mill motor input power insofar as it concernsthe rate and/or sense of change of the input power; translating thecontrol signal; and utilizing the translated signal to determine thecontrolling operation to be exercised on the mill for reducing the rateof change of mill motor input power towards zero.

The translated signal may be utilized to actuate automatic control meansfor the mill for reducing the rate of change of mill motor input powertowards zero. The automatic control means will determine and exerciseautomatically the required controlling operation on the mill and may beadapted to adjust the addition of grinding medium to the mill or to varythe rotational speed of the mill.

, The translated signal may also be utilized to actuate indicator means.If the mill is to be controlled manually an indication, such as forexample visual, can be obtained of whether the input power to the millmotor is increasngor decreasing at any instant, and the indicationobtained will determine the controlling operation to be exy, assautPatented sept. 13, lese ICC ercised on the mill for reducing the rate ofchange of mill motor input power towards zero.

According to another aspect of `the invention there is provided meansfor effecting operational control of a tumbling grinding mill driven byan electric motor, wherein variations in the volume of the grindingmedium in the mill cause variations in the input power to the millmotor, comprising a control signal generator in which an input signalderived from the power input to the mill motor and proportional to theinstantaneous mill motor input power, is interacted with a continuallyand automatically variable independently generated reference signal, andin which, on unbalance of the two signals due to an increase or decreaseof the mill motor input power, a control signal is generated which isindicative of the rate and sense of `change of the mill motor inputpower; control signal translating means; and means actuated `inaccordance with theintelligence conveyed by the translated signal fordetermining the controlling operation to be exercised on the mill forreducing the rate of change of mill motor input power towards zero; andreference signal Varying means actuated in accordance with theintelligence conveyed by the translated signal for restoring thereference signal towards constant balance with the input signal.

The means actuated in accordance with the intelligence conveyed by thetranslated signal for determining the controlling operation to beexercised on the mill may comprise automatic control apparatus or maycomprise indicator means.

Preferred embodiments of the invention will now be described purely byway of example and in no sense restrictive, with reference to theaccompanying drawings in which:

Figure 1 is a schematic representation of one system of effectingoperational control of a pebble mill.

Figure 2 is a circuit diagram of the apparatus for obtaining an outputsignal which is dependent on the change of input power to the millmotor.

Figure 3 is a front elevational view of the electrical switch means.

Figure `4 is a side elevational view of the electrical switch means.

Figure 5 is a circuit diagram of automatic control means for effectingautomatic operational control of a pebble mill.

Figure 6 is a schematic representation of an alternative system ofeffecting operational control of a pebble mill.

Figure 7 is a circuit diagram of alternative control means for effectingautomatic operational control of a pebble mill.

Figure 8 is a schematic representation of further alternative controlmeans for effecting automatic operational control of a pebble mill.

As shown in Figure 1 an input signal which is proportional to theinstantaneous input power to the mill motor is derived as 'analternating signal from current transformer 1 which is connected in themill motor power input circuit. After suitable step-up transformation bymeans of potential transformer 2, the alternating signal is rectified byrectier 3 and is then passed through an averaging network 4 with a longtime constant in order to minimize the effect of transient power surgeswhich are inherent in the operation of grinding mills.

The rectitied and averaged input signal is applied as a direct voltageto the signal generating bridge 5. Another direct voltage, obtained froma stabilized source and variable by means of potentiometer 10, isapplied to the bridge 5 as a reference signal.

The bridge 5 interactsor compares the Iinput and reference signals witheach other. The input signal varies with the mill motor input power andon imbalance of the two signals due to an increase or decrease of theinput power a control signal is generated which is responsive to thevariations of the mill motor input power, the magnitude and sense ofthe'control signal being indicative of the rate and sense of change ofmill motor input power.

The control signal is amplified by voltage amplifier 6 and phasesensitive power amplier 7 and is then fed to the control phase windingsof reversible two phase motor 8, the second phase windings of which arefed from an independent source. The reversible motor 8 translates thecontrol signal and the sense of rotation of motor 8 at any instant willdepend on the sense of the control signal at that instant and,therefore, on whether the input power to the mill motor is increasing ordecreasing at that instant.

The potentiometer 10, for varying the reference signal is rotatablycoupled to reversible motor 8 through reduction gearing. The setting ofpotentiometer 10 is varied automatically in accordance with theintelligence con- Veyed by the translated control signal in such amanner that the reference signal is restored continually towardsconstant balance with the input signal.

Magnetic clutch 9 is also rotatably coupled to the reversible motor 8through reduction gearing. The magnetic clutch 9 is adapted to operateelectrical switches 11 for indicator means 12 and/or automatic controlmeans 13. Indicator means 12 provides a visual indication of whether thepower input to the mill motor is increasing or decreasing at anyinstant, and where the mill is to be controlled manually, the indicationobtained will determine the controll-ing operation to be exercised onthe mill in order to restore the volume of grinding medium inthe milltowards the optimum value.

Control means 13 is arranged to determine and exercise automatically therequired controlling operation on the mill in o-rder to restore thevolume of grinding medium in the mill towards the optimum value.

The mill motor may be a 3 phase induction motor and as shown in Figure 2current transformer 1 is connected in one line of the rnill motor powerinput circuit. Over the operative region of the mill, power changes aresmall and to all intents and purposes `voltage and power factor remainconstant so that the input power to the mill motor can be lregarded asbeing proportional to the input current. Since the control signal is tobe a function of the change of input power to the mill motor, actualinstantaneous values of power or current are unimportant. The usualminor unbalances between the three phases of the input circuit will nothave any adverse eliects on the operation of the system and it issufficient to obtain the input signal from one phase only.

After suitable voltage step-up transformation in potential transformer 2the alternating input signal is applied to potentiometer 15 which isadjustable to provide control of the magnitude of the input signal. Theinput signal from potentiometer 15 is rectied in rectifier 3 and passedthrough the averaging network 4 comprising resistor 16 and condenserI17.

The signal generating bridge comprises twin triode 18 the cathodes ofwhich are connected together by means of the circuit containingcondenser 19 and the primary winding of transformer 20. The rectiiiedand averaged input signal is applied as a direct voltage to the one gridof twin triode 18. The reference signal is applied as a direct voltageto the other grid of twin triode 18.

The reference signal is `obtained from tube 21, to the plate of which analternating voltage from an independent source is applied via lead 22.The alternating voltage is rectified by tube 21 and is smoothed by meansof the iiltering circuit comprising choke 23 and condensers 24, 25 and26. The regulating tube 27 serves to stabilize the voltage which isvariable towards constant balance with the input signal by means ofpotentiometer 10.

Y A 50 cycle alternating voltage from an independent source is appliedto the plates of the twin triode 18 via lead 28. By virtue of theconnections of the twin triode 18 there is obtained from the secondarywinding of transformer 20 an alternating output control signal, themagnitude and sense of which is indicative of the rate and sense ofchange of the mill motor input power.

It will be appreciated that whereas the input signal is proportional tothe instantaneous Values of mill motor input power, the reference signalis in essence an integration of the input signal over a period of timestored and constantly compa-red with the instantaneous values of millmotor input power. In other words, the reference signal is proportionalat any instant to the average mill motor input power over a period oftime immediately preceding that instant.

The control signal is applied to the Vone grid of twin triode 30 of theVoltage amplifier 6 via lead 29. Twin triode 30 is connected to providetwo stages of voltage amplification and the output from the' second'stage Vis applied via lead 31 to phase sensitive power amplier 7comprising tubes 32 and 33. Tubes 32 and 33 are triodes connected with acommon grid bias cathode resistance, the grids being tied together. Twomore or less equal, 50 cycle, alternating voltages which are 180 out ofphase with each other are applied to the plates of tubes 32, 33 by meansof transformer 34 which is connected via leads 35, 36 to an independent50 cycle source of supply.

The output from power ampliiier tubes 32, 33 is obtained fromtransformer 34 via lead 37. The output from tubes 32, 33 will be inphase with the signal applied to them with the result that the amplifiedcontrol signal in lead 37 will have a sense which will be indicative ofthe sense of the change of input power to the mill motor.

The amplified control signal is fed viav lead 37 to th control phasewindings 38 of the reversible two phase motor 8. The second phasewinding 39 is fed from an independent source via leads 40, 41. Condenser42 is provided in lead 40, to cause a phase diierence between the powersupplied to the two windings 38, 39 so that two phase operation ispossible.

The magnetic clutch 9 and the electrical switches 11 which together formswitching means actuated by the. reversible motor 8, the potentiometer10` and the reversible motor 8 are arranged to form a single unit asshown in Figures 3 and 4.

The magnetic clutch 9 comprises a ferro-magnetic disc 43 which isprovided with a plurality of open slots 44 extending radially inwardsfrom the periphery. The slotted disc 43 is secured to shaft 45 which isrotatably supported by bearing 46 on support frame 47 and which isdriven from the internally geared reversible motor 8 through reductiongearing 48, 49.

Two pairs of rods 50, 51 and 52, S3 which Vare prok vided with threadedend portions are secured to bearing members 54 yand 55 respectively. Thebearing members 54 and 55 are supported on shaft 45 by means of bearingsin a manner allowing shaft 45 to rotate freely in the bearings withoutany appreciable rotational effect on members 54, 55.

Permanent magnets 56, 57 and 58, 59 are secured together in pairs withplates 60, 61 and 62, 63 respectively by means of nuts on screws 64, 65and 66, 67 respectively. The ends of screws 64, 65, 66, 67 are securedto the threaded ends of rods 50, 51, 52, 53 by means of nuts to form arrn support member which supports ,the`

magnets 56, 57 and 58, 59 along the periphery of slotted disc 43 inpositions which are more or less 180 apart.

The slotted disc 43 rotates in the field of the permanent magnets anddue to magnetic influence rotation ofthe slotted disc in a certain sensewill cause the support mem-1 ber to be pivoted in one or the otherdirection.

The electrical switches 11 are operated bythe magnetic clutch throughmediumof Va pivoted member 68- provided with engaging portion 69extending upwards between nuts 70, 71 on screws 64, 65 and with armportions 72 and 73. Movement of the support member in the one directionor the other will cause engaging portion 69 to be engaged by either nut70 or nut 71 as the case may be. This will cause the pivoted member 68to pivot around its pivot 74 and assume one of two positions. Dependingportions 75 and 76 are provided on member 68 to act as stop members tolimit the pivotal movement of member 68 and hence also to limit thepivotal movement of the support member.

Electrical switches which are generally denoted by reference numeral 11are mounted on the pivoted member 68 and will open or close depending onthe position assumed by pivoted member 68. Two specific mercury switches77 and 78 are shown mounted on arm portions 72 and 73 respectively, insuch a manner that when the pivoted member 68 assumes one of its twopositions, the one mercury switch will close while the other will open.When the pivoted member 68 assumes the other position the closing andopening of the switches will be reversed.

It will be seen that the switching means described is so arranged thatonce the pivoted member 68 has assumed one of its two positions ongeneration of a control signal of a certain sense, the pivoted member 68will remain in that position until a control signal of opposite sense isgenerated, which will then cause the pivoted member 68 to assume itsother position.

` The two mercury switches 77 and 78 can be adapted to actuate theindicator means 12 to provide a visual indication of whether the inputpower to the mill motor is increasing or decreasing at any instant. Themercury switches 77 and 78 can also be adapted to actuate the automaticcontrol means 13 It will be appreciated that the sense of rotation ofthe magnetic disc 43 provides a very convenient visual indication of thechange of mill motor input power.

The potentiometer for varying the reference signal is driven from motor8 through reduction gearing 48, 49, 79a, 79b, 79e.

In this specification the term pebble mill designates a tube mill usinga coarse component of ore as grinding medium. Such a mill normallyreceives two components of feed, namely coarse ore pebbles acting asgrinding medium and fine ore. No extraneous grinding medium need beused. The coarse ore pebbles and the tine ore may be fed separately tothe mill. Alternatively, the feed to the mill may comprise essentiallyuncrushed runof-mine ore containing both coarse ore pebbles and fineore.

The automatic control means 13 can be adapted to restore the mill loadcontinually towards the optimum value in order to bring the mill motorinput power towards maximum by arranging for a pebble feeding operationto the mill to be arrested when the intelligence conveyed by thetranslated control signal indicates a decrease of input power to themill motor and for another pebble feeding operation to be initiatedsubsequently when the intelligence conveyed by the translated signalindicates a change in input power in the opposite sense. The decrease ofmill motor input power will be due to the mill load exceeding theoptimum value and with this type of control there will be a tendency forthe mill to operate with a slight overload.

At or near maximum power input to the mill motor, changes in power for agiven increment in load are small and in order to prevent ambiguityarising when the intelligence conveyed by a translated control signalindicates a decrease of mill motor input power, the control yapparatus13 can be so arranged that during successive periods of time, the pebblefeeding operation is alternately compulsory and optional. Thearrangement can further be such that pebble feeding can be arrested atany time during the peri-od when pebble feeding is optional.

As shown in Figure 5 a D C. plate voltage is provided for thyratron tubeby means of transformer 81, rectifier 82 and the smoothing circuitcomprising resistor 83 and condensers 84, 85. By means of the circuitcomprising resistors 86, 87, 88, rectifier 89 and condenser 90 the gridof thyratron 80 is held suliiciently negative to prevent thyratron 80from conducting when the plate circuit is completed. Switch 77 is thatmercury switch which is closed when the intelligence conveyed by atranslated signal indicates a decrease of input power to the mill motor.When switch 77 is closed the grid of thyratron 80 is earthed and it willbe possible for the thyratron 80 to conduct if its plate circuit iscompleted. Relay 9.1" is energized by the current in the plate circuitwhen the thyratron 80 conducts.

An automatic timer device 92 is provided to alternately complete andinterrupt the plate circuit of thyratron 80 during successivepredetermined periods of time by means of switch 93. The timer device 92may comprise a cam which is driven by a synchronous motor. The cam canbe so arranged that it interrupts the plate circuit during apredetermined part of its rotational cycle.4

Any other suitable timer device may be utilized.

At any time during the period when the plate circuit is completed, thethyratron 80 can start to conduct if switch 77 is closed to earth thegrid. Once conduction has commenced it will continue until the platecircuit is interrupted again by the timer device.

Conduction of the thyratron 80 will energize relay 91 which will in turnenergize a normally closed relay in the starting circuit 94 of thepebble feeder motor to arrest the pebble feeding operation. When theplate circuit of thyratron 80 is interrupted, the relays will deenergizeand another pebble feeding operation will be initiated.

The duration of the alternate compulsory :and optional peeble feedingperiods can be adjusted to suit the operational conditions ofthe mill.

In the system just described the control signal is translated by meansof a reversible electric motor. Numerous other phase sensitive deviceswhich will be responsive to the control signal can be used for the samepurpose. A signal responsive relay, such as for example a differentialrelay provided with coils containing bucking windings, may be used andFigure 6 shows schematically an alternative system of eliectingoperational control of a mill in which a signal responsive relay isincorporated.

The amplified control signal is obtained as described above and thereference signal is again varied automatically by means of thepotentiometer 10 which is driven by reversible motor 8. The magneticclutch 9 is, however, dispensed with a differential relay 95 whichoperates electrical switches is provided to translate the controlsignal. The electrical switches will actuate the indicator and/orautomatic control means. In this embodiment of the invention the senseof rotation of the reversible motor will provide a convenient visualindication of the change of mill motor input power.

The operational control of the mill described above with reference toFigure 5 -adjusts the addition of pebbles to the mill. The addition ofpebbles may be controlled in a variety of ways and it is also possibleto effect operational control by varying the rotational speed of themill.

An alternative way of controlling the addition of pebbles to the mill inorder to bring the mill motor input power towards maximum, can beachieved by arranging for a pebble feeding operation to the mill to beinitiated when the intelligence conveyed by the translated signalindicates a decrease of input power to the mill motor and for the pebblefeeding operation to be arrested subsequently when the intelligenceconveyed by the translated signal indicates a change of input power inthe opposite sense. In this case the decrease of mill motor input powermust be due to the mill load falling short of the optimum value andthere will be a tendency for the mill to operate with a load slightlyless than the optimum.

' The control apparatus 13 may be so arranged that during successiveperiods of time the pebble feeding operation is alternately possible andprecluded and that pebble feeding can be initiated at any time duringthe period when the pebble feeding operation is possible.

The circuit shown in Figure can be used for this type of control. Theoperation is exactly the same as described before with the exceptionthat a normally open relay is provided in the starting circuit 94 of thepebble feeder motor. A pebble feeding operation is precluded during theperiod when the plate circuit of thyratron 80 is interrupted and ispossible during the period when the plate circuit is closed. If switch77 is closed a pebble feeding operation will be initiated during aperiod when the plate circuit of the thyratron 80 is closed. Once peeblefeeding has commenced it will continue -until the plate circuit isinterrupted again.

The duration of the alternate periods during which [pebble feeding ispossible and precluded can be adjusted to suit the operationalconditions of the mill.

As a further alternative the control means 13 can be arranged to arresta pebble feeding operation to the mill when the intelligence conveyed bythe translated signal indicates a decrease of input power to the millmotor due to the mill load exceeding the optimum value, and subsequentlyto initiate another pebble feeding operation when the intelligenceconveyed by the translated signal indicates a decrease of input powerdue to the mill load falling short of the optimum value.

It will be appreciated that in this embodiment, means must be providedfor distinguishing between a decrease of input power due to the millload exceeding the optimum value and a decrease of input power due tothe mill load falling short of the optimum value. This can be achievedby arranging for a predetermined time interval to elapse after thepebble feeding operation has been arrested before a subsequent operationcan be started, and similarly for a predetermined time interval toelapse after a pebble feeding operation has lbeen initiated before itcan be arrested again.

The circuit shown in Figure 5 can be used for effecting this type ofcontrol. The timer device 92 can be arranged to provide the timeintervals required and the relays in the feeder motor circuit 94 can beadapted to initiate and arrest feeding operations as required under theinfluence of relay 91.

As yet another alternative, the control means 13 may be arranged to varythe rate of pebble feed to the mill in accordance with the rate andsense of change of mill motor input power.

Where a D.C. motor is used to drive the pebble feeder, the circuitarrangement shown in Figure 7 may be used to effect this type ofcontrol. The control signal is utilized as before to cause rotation ofreversible motor 8 which will in turn cause magnetic clutch 9 tooperate. The mercury switches operated by the magnetic clutch arearranged to form a double pole, double throw changeover switch asrepresented by 96 in Figure 7. A control signal conveying theintelligence of an increase of input power to the mill motor will causeswitch 96 to assume one switching position and a control signalconveying the intelligence of a decrease of input power will causeswitch 96 to assume the other switching position.

By means of lead 97, the output control signal from the second stage ofthe twin triode 30 (see Figure 2) is applied to the grid of tube 98which receives its plate supp-ly via lead 99. The signal is obtained asan amplified, iiuctuating7 direct voltage from the plate circuit of tube98 and is fed to the change-over switch 96. The signal may be smoothedto reduce the fluctuations.

The signal is fed via the change-over switch 96 to auxiliary excitationwindings A of the feeder `motor 94. Depending on the switching positionof switch 96the excitation produced by winding 100 will either assist oroppose the excitation produced bythe main excitation winding 101. Theextent of the vopposition or assistanceY will depend on the -rate andsense of change of mill motor input power, and the rate of pebble feedwill be varied" accordingly.

yThe methods of controlling the feed of pebbles to a pebble mill asdescribed above may be used for controlling the addition of grindingmedium to any tumbling mill using a coarse component of the ore asgrinding medium.A These methods may be used for the control of tumblingmills receiving feed consisting of essentially uncrushedv run-of-mineore as well as for the control of the addition of fine feed to tumblingmills.

Under certain circumstances it might be more advantageous to vary therotational speed of the mill and theY control means 13 can also beadapted for that purpose.

Where an A.C. motor with a wound rotor is used as a mill motor, therotational speed may, for example, be varied by using a steppedresistance for controlling the motor. A translated control signalconveying the intelligence of either an increase or decrease of inputpowerl to the mill motor can then be utilized to adjust the steps of theresistance in order to increase or decrease the rotational speed of themill motor as required. i Y It is possible to effect operational controlof a mill by controlling both the addition of feed to the mill and therotational speed of the mill. Y It may be advantageous to arrange thecontrol meansV 13 so that the rotational speed of the mill motor willVnot be varied when casual changes of the mill motor? input power occur,but will be varied only when a sustained change occurs. An example ofthis type of control will be described with reference to a pebble millin which the rotational speed is variable and the pebble feedingoperation is controlled by arresting the pebble feeding operation whenthe intelligence conveyed by the translated control signal indicates adecrease of inputpower due to overloading of the mill. Figure 8 showsthe arrangement schematically. A stepped relay 103 which is adapted toperforml successive operations on receipt of successive actuatingsignals is provided in circuit with the control resistance 104 of themill motor 105 and is also associated with the'v plate circuit of thethyratron tube 80 in circuit 102 which is similar to the circuit shownin Figure 5. When the translated control signal conveys the intelligenceof av decrease of input power to the mill motor, the pebblefeedingoperation is arrested during the first period when pebble feeding isoptional and an actuating signal of a certain sense is transmitted tothe stepped relay 103. An actuating signal of similar sense will betransmitted to the stepped relay 103 each time the pebble feedingoperation is arrested. Generation of a control signal which indicates anincrease of mill motor input power and which will result in the pebblefeeding operation Ibeing maintained during at least part of the optionalperiod, will result in an actuating signal of opposite sense beingtransmitted to the stepped relay 103. The stepped relay 103 is soarranged that two successive actuating signals of the same sense arerequired before it can actuate the variation of the setting of thecontrol resistance 104 of the mill motor 105. Each successive actuatingsignal of the same sense after the first causes the control resistance104 to be adjusted one step andL the rotational speed of the mill motor105 tobe varied accordingly. After a number of actuating signals of thesame sense have been received in succession, an actuating signal ofopposite sense will cancel the preceding signals. but will not cause thesetting of control resistance 104 to be varied. Only upon receipt of asecond successive signal of the same sense will the control resistance104v be adjusted one step in the direction opposite to the previousdirection of adjustment.

In the embodiments described above the control signal is electrical inits nal form, but it may also be converted into either a pneumatic orhydraulic signal for eifecting the operational control of the mill.

I claim:

1. Means for effecting automatic operational control of a tumblinggrinding mill driven by an electric motor, wherein variations in thevolume of the grinding medium in the mill cause variations in the inputpower to the mill motor, comprising a control signal generator in whichan input signal derived from the power input to the mill motor andproportional to the instantaneous mill motor input power, is comparedwith an independently generated reference signal Awhich is continuallyand automatically variable towards constant balance with the inputsignal, and in which control signal generator, on unbalance of the twosignals due to an increase or decrease of the mill motor input power, acontrol signal is generated which is indicative of the rate land senseof change of the mill motor input power; control signal translatingmeans; automatic control apparatus for the mill actuated in accordancewith the intelligence conveyed by the translated signal for `bringingthe power input to the mill motor continually towards a maximum; andreference signal varying means actuated in `accordance with theintelligence conveyed by the translated signal for restoring thereference signal towards constant balance with the input signal.

2. Means for effecting automatic operational control of a tumblinggrinding mill driven by an electric motor, wherein variations in thevolume of the grinding medium in the mill cause variations in the inputpower to the mill motor, comprising a control signal generator in whichan input signal derived from the power input to the mill motor andproportional to the instantaneous mill motor input power is comparedwith an independently generated reference signal which is continuallyand automatically variable towards constant balance with the inputsignal, and in which the control signal generator, on unbalance of thetwo signals due to an increase or decrease of the mill motor inputpower, a control signal isl generated which is indicative of the rateand sense of change of the mill motor input power; control signaltranslating means; automatic control apparatus for the mill actuated inaccordance with the intelligence conveyed `by the translated signal forbringing the power input to the mill motor continually towards amaximum; and reference signal varying means actuated in accordance withthe intelligence conveyed by the translated signal for restoring thereference signal towards constant balance with the input signal, theautomatic control apparatus comprising an electronic circuit including athyratron tube, an electrical control switch actuated in accordance withthe intelligence conveyed by the translated signal, and provided in thegrid circuit of the thyratron tube; a timer provided in circuit with theplate of the thyratron tube and adapted to cause the thyratron `platecircuit to be alternately .completed and interrupted during successivepredetermined periods of time; and a relay provided in the thyratronplate circuit and adapted to cause interruption and completion of thestarting circuit of an electric motor driving the grinding medium`feeder of the mill.

3. Means for effecting automatic operational control of a tumblinggrinding mill driven by an electric motor, wherein variations in thevolume of the grinding medium in the mill cause variations in the inputpower to the mill `motor, comprising `a control signal generator inwhichlan input signal derived from the power input to the mill motor andproportional to the instantaneous mill motor input power is comparedwith an independently generated reference signal which is continuallyand automatically variable towards constant balance with the if) inputsignal, and in which the control signal generator, on unbalance of thetwo signals due to an increase or decrease of the mill motor inputpower, a control signal is generated which is indicative of the rate:and sense of change of the mill motor input power; control signaltranslating means; automatic control apparatus for the mill actuated inaccordance with the intelligence conveyed by the translated signal forbringing the power input to the mill motor continually towards amaximum; and reference signal varying means actuated in accordance withthe intelligence conveyed by the translated signal for restoring thereference signal towards constant balance with the input signal, theautomatic control apparatus comprising an electronic circuit including athyratron tube, an electrical control switch actuated in accordance withthe intelligence conveyed by the translatedV signal and provided in thegrid circuit of the thyratron tube; a control resistance for the millmotor; and a stepped relay connected to the control resistance andassociated with the plate circuit of the thyratron tube.

4. Means for effecting automatic operation control of a tumblinggrinding mill driven by an electric motor, wherein variations in thevolume of the grinding medium in the mill cause variations in the inputpower to the mill motor, comprising a control signal generator in whichan input signal derived from the power input to the mill motor andproportional to the instantaneous mill motor input power, is comparedwith an independently generated reference signal which is continuallyand automatically variable towards constant balance with the inputsignal, and in which the control signal generator, on unbalance of thetwo signals due to an increase or decrease of the mill motor inputpower, a control signal is generated which is indicative of the rate andsense of change of the mill motor input power; control signaltranslating means; automatic control apparatus for the mill actuated inaccordance with the intelligence conveyed by the translated signal forbringing the power input to the mill motor continually towards amaximum; and reference signal varying means actuated in accordance withthe intelligence conveyed by the translated signal for restoring thereference signal towards constant balance with the input signal, theautomatic control apparatus comprising a circuit including a change-overswitch actuated in accordance with the sense of change of mill motorinput power conveyed by the translated signal; and auxiliary excitationwindings provided in an electric moto-r driving the grinding mediumfeeder of the mill and adapted to be energised via the change-overswitch in accordance with the rate and sense of change of mill motorinput power.

5. Means for effecting automatic operational control of a tumblinggrinding mill driven by an electric motor, wherein variations in thevolume of the grinding medium in the mill cause variations in the inputpower to the mill motor, comprising a control signal generator in whichan input signal derived from the power input to the mill motor andproportional to the instantaneous mill motor input power, is comparedwith an independently generated reference signal which is continuallyand automatically variable towards constant balance with the inputsignal, and in which the control signal generator, on unbalance of thetwo signals due to an increase or decrease of the mill motor inputpower, a control signal is generated which is indicative of the rate andsense of change of the mill motor input power; control signaltranslating means; automatic control apparatus for the mill actuated inaccordance with the intelligence conveyed by the translated signal forbringing the power input to the mill motor continually towards amaximum; and reference signal Varying means actuated in accordance withthe intelligence conveyed by the translated signal for restoring thereference signal towards constant balance with the input signal and inwhich the control signal translating means comprises a reversible,signal 11 responsive electric motor including control windings to whichthe control signal is applied; and comprising electric switching meansresponsive to the reversible motor, the electric switching meansincluding a circumferentially slotted disc of ferro-magnetic material,which is rotatably driven by the reversible electric motor; a supportmember adapted for limited pivotal movement co-axially with the slotteddisc; at least one magnet secured to the support member and adapted toproduce a magnetic field in which the slotted disc rotates to causepivotal movement of the support member in one or the other directiondepending on the sense of rotation of the slotted disc; a pivoted memberprovided with a portion adapted to engage with the support member,movement of the support member in one or the other direction causing thepivoted member to assume one of two positions; and at least oneelectrical control switch which is mounted on the pivoted member, isconnected in the circuit with the automatic control apparatus and isarranged to close when the pivoted member assumes one position and toopen when the pivoted member assumes the other position.

6. A method of effecting automatic operational control of a tumblinggrinding mill driven by an electric motor, wherein variations in thevolume of grinding medium in the mill causes variations in the inputpower to the mill motor, comprising producing an input signal which isproportional to the instantaneous input power to the mill motor,producing a reference signal which is proportional at any instant to theaverage mill motor input power over a period of time immediatelypreceding that instant; comparing the two signals with each other toproduce a control signal which is indicative of the rate and sense ofchange of the mill motor input power, when unbalance occurs between theinput and reference signals due to an increase or decrease of the millmotor input power; translating the control signal and controlling themill in accordance with the intelligence conveyed by the translatedsignal to bring the power input to the mill continually towards amaximum.

7. A method of effecting automatic operational control of a tumblinggrinding mill driven by an electric motor, wherein variations in thevolume of grinding medium in the mill causes variations in the inputpower to the mill motor, comprising producing an input signal which isproportional to the instantaneous input power to the mill motor,producing a reference signal which is continually and automaticallyvariable towards constant balance with the input signal; comparing thetwo signals with each other to produce a control signal which isindicative of the rate and sense of change of the mill motor inputpower, when unbalance occurs between the input and reference signals dueto an increase or decrease of the mill motor input power; translatingthe control signal; restoring the reference signal towards constantbalance with the input signal in accordance with the intelligenceconveyed by the translated signal; and controlling the mill inaccordance with the intelligence conveyed by the translated signal tobring the power input to the mill continually towards a maximum, saidmethod further comprising adjusting the addition of grinding medium tothe mill in -accordance with the intelligence conveyed by the translatedsignal to reduce the rate of change of mill motor input power towardszero, the addition of grinding medium to the mill being alternatelycompulsory and optional during successive periods of time; the operationof addition of grinding medium to the mill being continued during anoptional period it during that period the intelligence conveyed by thetranslated signal indicates that the mill motor input power isincreasing, the operation of addition of grinding medium being arrestedat any instant during an optional period and being precluded from beinginitiated again during the remainder of that period if the intelligenceconveyed by the translated signal indicates that theuinput power isdecreasing.

8. A method as claimed in claim 7, in which the grindring medium formspart of the feed to the mill whichl comprises essentially uncrushedrun-of-mine ore.

9. A method of effecting automatic operational control of a tumblinggrinding mill driven by an electric motor, wherein variations in thevolume of grinding medium in the mill causes variations in the inputpower to the mill with each other to produce a control signal which isindicative of the rate and sense of change of the mill motor inputpower, when unbalance occurs between the input; and reference signalsdue to an increase or decrease of the mill motor input power;translating the control signal; restoring the reference signal towardsconstant balance with the input signal in accordance with theintelligence conveyed by the translated signal; and controlling the millin accordance with the intelligence con-` veyed by the translated signalto bring the power input to the mill continually towards a maximum, theaddition' of grinding medium to the mill being adjusted in accordancewith the intelligence conveyed by the translated signal to reduce therate of change of mill motor input power towards zero, the addition ofgrinding medium to the mill being alternately possible and precludedduring successive periods of time; preclusion of an operation ofaddition of grinding medium to the mill being continued during a periodwhen an operation of addition is possible if during that period theintelligence conveyed by the translated signal indicates that the millmotor input power is increasing, an operation of addition of grindingmedium `being initiated at any instant during a period when an operationof addition is possible and being maintained for the remainder of thatperiod if the intelligence conveyed by the translated signal indicatesthat the input power is decreasing.

10. A method of effecting automatic operational control of a tumblinggrinding mill driven by an electric motor, wherein variations in thevolume of grinding medium in the mill causes variations in the inputpower to the mill motor, comprising producing an input signal which isproportional to the instantaneous input power to the mill motor,producing a reference signal which is continually and automaticallyvariable towards constant balance with the input signal; comparing thetwo signals with each other to produce a control signal which isindicative of the rate and sense of change of the mill motor inputpower, when unbalance occurs between the input and reference signals dueto an increase or decrease of the mill motor input power; translatingthe controll signal; restoring the reference signal towards constantbalance with the input signal in accordance with the intelligenceconveyed by the translated signal; and controlling the mill inaccordance with the intelligence conveyed by the translated signal tobring the power input to the mill continually towards a maximum, theadditiony of grinding medium to the mill being adjusted in accordancewith the intelligence conveyed by the translated signal to reduce therate of change of mill motor input power towards zero, an operation ofladdition of grinding medium to the mill being arrested if theintelligence conveyed by the translated signal indicates that the millmotor input power is decreasing, and another operationY of addition ofgrinding medium being subsequently in? itiated if the intelligenceconveyed by the translated signal again indicates that the power isdecreasing. A

11. A method as claimed in claim 10, in which a pre? determined timeinterval is arranged to elapse after anY operation of addition ofgrinding medium is arrested before a subsequent operation of additioncan be in? 13` itiated, and in which a predetermined time interval isarranged to elapse after an operation of addition is initiated before itcan be arrested again.

12. A method of eiecting automatic operational control of a tumblinggrinding mill driven by an electric motor, wherein variations in thevolume of grinding medium in the mill causes variations in the inputpower to the mill motor, comprising producing an input signal which isproportional to the instantaneous input power to the mill motor,producing a reference signal which is continually and automaticallyVariable towards constant balance with the input signal; comparing thetwo signals with each other to produce a control signal which isindicative of the rate and sense of change of the mill motor inputpower, when unbalance occurs between the input and reference signals dueto an increase or decrease of the mill motor input power; translatingthe control signal; restoring the reference signal towards constantbalance with the input signal in accordance with the intelligenceconveyed by the translated signal; and controlling the mill inaccordance with the intelligence conveyed by the translated signal tobring the power input to the mill continually towards a maximum, thefeed to the mill comprising essentially uncrushed run-of-mine ore andthe coarse component acting as the grinding medium, the feed being addedto the mill continuously and the rate of addition being varied inaccordance with the intelligence conveyed -by the translated signal toreduce the rate of change of mill motor input power towards zero.

References Cited n the le of this patent UNITED STATES PATENTS 1,395,089Burhans Oct. 25, 1921 1,619,807 Blomeld Mar. 8, 1927 2,766,939 WestonOct. 16, 1956 2,766,940 Weston Oct. 16, 1956 2,766,941 Weston Oct. 16,1956 FOREIGN PATENTS 475,421 Great Britain Nov. 18, 1937

